Teams aren’t always concerned about winning friendlies as there’s no trophy nor title to be won and I don’t think it really affects their rankings. I haven’t placed a bet and don’t think I’ll be moved anytime soon to place bets on a friendly match. Unless, they’re actually trying to win the game, I don’t see any team using their first choice of players for a friendly game. They’re more likely to use perhaps a mixture of players who enjoys playing time and others who don’t normally have a lot of playing time.
Any outcome (even if we are talking about a normal game) is difficult to predict, but this is not required: you just need to understand where the value is. In friendly games where there is an obvious favorite, you should bet on the outsider (even taking into account the fact that bookmakers usually adjust the value odds on this side), if approximately equal teams play, then (this is my personal opinion, not financial advice hehe) the value is in BTTS No (since everyone expects that there will be BTTS Yes).
In general, friendly games are a very random topic, and in order to make a profit from it, you need to place live bets.